docs: add offline agent policy server evidence

[JOY (JOY)]

Summary

• Add Implement offline-agent policy UI and return report #149 evidence pack for the existing Nakama offline-agent policy and return-report server slice.
• Document policy modes, validation, low-BodyTime downgrade, dead-body rejection, report rows, activity/ledger correlation, and authority boundaries.
• Keep Unity policy UI, return report UI, and full overnight autonomy explicitly open.

Verification

• npm.cmd ci
• npm.cmd run build
• npm.cmd test
• git diff --check
• rg -n "Not run|TODO|TBD|—|–" docs/playtests/2026-05-30-offline-agent-policy-server-evidence returned no matches
• npx.cmd markdownlint-cli2 docs/playtests/2026-05-30-offline-agent-policy-server-evidence/*.md

Local review fallback

• Verdict: approved for PR.
• Docs-only evidence update, no runtime behavior changed.
• Checked authority boundary: evidence maps to existing Nakama tests and does not claim Unity UI completion.
• Checked Unity scope: no Unity scenes, prefabs, packages, materials, or ProjectSettings changed.

Refs #149